This invention relates to a furnace and method for the sintering of refractory and ceramic materials using plasma heated gases.
Prior art sintering furnaces, in general, tend to be inefficient and slow. Long furnace retention times are necessary when using conventional sintering furnaces, which results in poor energy utilization, excessive furnace gas consumption and high maintenance costs.
Many ceramic or refractory materials are sintered in prior art tunnel or periodic kilns which are fired by energy released from the combustion of fossil fuels with air or oxygen. If the ceramic or refractory material can be exposed to air and/or the products of combustion, then the kiln may be directly fired, in which case, the heating and utilization of energy may be reasonably efficient. However, for certain refractory materials, such as the carbides, nitrides and borides, the firing must be done in the absence of oxygen or oxygen-bearing gases, including water and carbon dioxide, to prevent formation of oxides, which may result in products having undesirable physical and chemical properties. Under such conditions, fossil fuel-fired furnaces may be used but the ceramic or refractory materials must be kept in a controlled environment, such as a retort, isolated from the combustion products of the fuel. Such heating is indirect, inefficient and slow. On a commercial scale, an apparatus such as a tunnel kiln requires about 70-90 hours (including the cooling cycle) to sinter refractory or ceramic articles.
Prior art electric kilns are also used to sinter ceramic or refractory materials under controlled atmospheres, but again tend to be energy inefficient and slow. In the case of a kiln equipped with graphite heating elements, the voltage can be controlled and the kiln can be heated to fairly high temperatures, yet there are several disadvantages: (1) The heating elements have a limited size, complex shape and must be kept under a strictly controlled atmosphere to maintain a long life; and (2) Furnace size is limited and it is difficult to achieve a uniform temperature in this type of kiln because the heating elements provide only radiant heat. Because of radiant heat transfer, as well as a heat element size limit, the kiln has a poor load density, a limited productivity and a poor energy efficiency. A typical sintering cycle time using a prior art electric kiln is around 24 hours (including cooling).
Plasma arc technology has recently been applied to the production of refractory and ceramic materials to reduce the furnace energy requirements and retention times. Plasma sintering of refractory and ceramic materials results in higher density and superior strength products than those made by conventional processes.
Plasma arc fired gases differ greatly from ordinary furnace heated gases in that they become ionized and contain electrically charged particles capable of transferring electricity and heat; or, as in the case of nitrogen, become dissociated and highly reactive. For example, nitrogen plasma gas dissociates into a highly reactive mixture of N.sub.2 -molecules, N-atoms, N.sup.+ -ions and electrons. This dissociation or ionization greatly increases the reaction rates for sintering ceramic or refractory materials. Nitrogen, for example, which dissociates at around 5000.degree. C. and 1 atmosphere pressure, would not dissociate under the normal furnace sintering conditions of around 1500.degree. C.-2000.degree. C. Thus, the use of plasma gases results in a highly reactive environment, which greatly increases the reaction sintering rate.
Plasma arc technology has generally only been used for the fusion of high temperature materials and not for sintering or reaction sintering. This is because the required sintering temperature for most ceramic or refractory materials is usually less than 2500.degree. C., whereas the average temperature of gases heated with a plasma arc torch is above 4000.degree. C. At such high temperatures, the refractory or ceramic materials may decompose. However, a plasma gas can be superheated to effect ionization or dissociation, while the ceramic or refractory material is then directly heated by this preheated gas to a much lower temperature. For example, nitrogen plasma gas heated to around 3000.degree. C. will bring silicon carbide refractory articles up to a temperature of 1000.degree. C.-1600.degree. C. in two to eight hours; and nitrogen plasma gas heated to around 4000.degree. C. will bring the articles up to a temperature of 1900.degree. C.-2200.degree. C. in the same time period. Thus, a plasma gas may be heated to a much higher temperature than the sintering temperature required.